Plasma-assisted ammonia synthesis in a packed-bed dielectric barrier discharge reactor: roles of dielectric constant and thermal conductivity of packing materials

In this article, plasma-assisted NH₃ synthesis directly from N₂ and H₂ over packing materials with different dielectric constants (BaTiO₃, TiO₂ and SiO₂) and thermal conductivities (BeO, AlN and Al₂O₃) at room temperature and atmospheric pressure is reported. The higher dielectric constant and thermal conductivity of packing material are found to be the key parameters in enhancing the NH₃ synthesis performance. The NH₃ concentration of 1344 ppm is achieved in the presence of BaTiO₃, which is 106% higher than that of SiO₂, at the specific input energy (SIE) of 5.4 kJ·l^-1. The presence of materials with higher dielectric constant, i.e. BaTiO₃ and TiO₂ in this work, would contribute to the increase of electron energy and energy injected to plasma, which is conductive to the generation of chemically active species by electron-impact reactions. Therefore, the employment of packing materials with higher dielectric constant has proved to be beneficial for NH₃ synthesis. Compared to that of Al₂O₃, the presence of BeO and AlN yields 31.0% and 16.9% improvement in NH₃ concentration, respectively, at the SIE of 5.4 kJ·l^-1. The results of IR imaging show that the addition of BeO decreases the surface temperature of the packed region by 20.5% to 70.3℃ and results in an extension of entropy increment compared to that of Al₂O₃, at the SIE of 5.4 kJ·l^-1. The results indicate that the presence of materials with higher thermal conductivity is beneficial for NH₃ synthesis, which has been confirmed by the lower surface temperature and higher entropy increment of the packed region. In addition, when SIE is higher than the optimal value, further increasing SIE would lead to the decrease of energy efficiency, which would be related to the exacerbation in reverse reaction of NH₃ formation reactions.